Transmissions that shift gears by engaging one clutch mechanism while simultaneously disengaging another clutch mechanism are well known in the art. Tractors and the like, which operate under heavy loads, are known to include transmissions having several powershift clutches which are selectively operated by a single shift lever. Such clutches are arranged in series such that a sequence of successively higher gear ratios, from neutral to an Nth speed, are obtained as the shift lever is selectively moved from one position to another. Such transmissions usually develop relatively high torque and relatively low speed outputs.
Many of such transmissions have hydraulically actuated powershift clutches which operate in response to a pressurized fluid flow thereto. Unless fluid flow to each respective clutch is carefully regulated and controlled, however, such clutches impart rough shifts between gear ratios. Such rough shifts can jolt the vehicle, particularly if a shift is made under load.
To avoid such shifting problems between gear ratios, various systems have been proposed for controlling operation of the clutches. Such control systems usually include a series of spool valves disposed between a source of hydraulic fluid pressure and the transmission clutches. As is known, pressurized fluid flow to each clutch mechanism is determined as a function of the linear disposition of the particular spool valve associated with that clutch.
In one type of control system, each spool valve is reciprocally arranged in a valve body. Depending upon valve body design, each spool valve assumes two or more linear positions within the valve body. To move a spool valve from one position to another within the valve body, it is common to use an electric solenoid capable of producing a fluid output in response to a control signal being delivered to the solenoid. The fluid output of the solenoid is delivered to one end of an associated valve spool in a manner regulating its linear disposition within the valve body. The valve spool is usually returned to its initial position under the influence of a spring.
Alternatively, two electric solenoids, each capable of producing a fluid output in response to a control signal being delivered thereto, are associated with each valve. The fluid output of one electric solenoid controls movement of the spool valve in one linear direction. The fluid output of the other solenoid controls movement of the spool valve in the opposite linear direction. As will be understood, two electrically controlled, fluid output solenoids provide a more positive and generally quicker spool valve operation.
Other control systems are designed such that one or more reciprocal spool valves assume three different operable positions for controlling fluid flow through the valve body. A spool valve which is positioned in any of three operable positions usually has two electrically controlled, fluid output solenoids and a spool centering spring associated therewith. The fluid output of one solenoid moves the spool valve to one operable position. The output of the other solenoid moves the spool valve to its second operable position. The spool centering spring locates the spool valve in its third operable position.
Therefore, a control system having a valve body with three reciprocal spool valves, each of which assumes one of three operable positions, would require six solenoids along with three spool centering springs. In addition to the foregoing, electronics including logic circuitry for controlling operation of each solenoid would also be required.
Because of operating clearances within each valve spool and tolerance accumulations associated with each electric solenoid, synchronization of the three spool valves in such a control system can, at best, only be approximated. Moreover, if any of the electric solenoids or spool valves become "stuck" during their operation, severe damage may be incurred within the transmission.